Heart disease is the leading cause of death in the United States. A heart attack (also known as an acute myocardial infarction (AMI)) typically results from a thrombus (i.e., a blood clot) that obstructs blood flow in one or more coronary arteries. AMI is a common and life-threatening complication of coronary artery disease. Myocardial ischemia is caused by an insufficiency of oxygen to the heart muscle. Ischemia is typically provoked by physical activity or other causes of increased heart rate when at least one coronary artery is narrowed by atherosclerosis. Patients will often (but not always) experience chest discomfort (angina) when the heart muscle is experiencing ischemia. Those with coronary atherosclerosis are at higher risk for AMI if the plaque becomes further obstructed by thrombus. Those patients who do not have any symptom of ischemia or AMI are said to have “silent ischemia.” These patients are at the highest risk of dying from coronary artery disease.
The current treatment for a coronary artery narrowing (a stenosis) is the insertion of a drug eluting stent such as the Cypher™ sirolimus-eluting stent from Cordis Corporation or the Taxus™ paclitaxel-eluting stent from the Boston Scientific Corporation. The insertion of a stent into a stenosed coronary artery is the most reliable medical treatment to eliminate or reduce coronary ischemia and to prevent the complete blockage of a coronary artery, which complete blockage results in an AMI.
Acute myocardial infarction and ischemia may be detected from a patient's electrocardiogram (ECG) by noting an ST segment shift (i.e., voltage change) over a relatively short (less than 5 minutes) period of time after a complete blockage of a coronary artery. However, without knowing the patient's normal (i.e., baseline) ECG pattern, detection from a standard 12 lead ECG can be unreliable.
Fischell, et al in U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023 describe implantable systems and algorithms for detecting the onset of acute myocardial infarction and providing both treatment and patient alerting. While Fischell, et al discuss the acute detection of a shift in the ST segment of the patient's electrogram from an electrode within the heart as the trigger for alarms, it may be desirable to provide more sophisticated long term tracking of myocardial ischemia to provide early prediction of coronary obstruction before the occurrence of a complete coronary artery blockage that results in an AMI. An important aspect of the Fischell, et al patents is that the heart's electrical signal from inside the patient's body, which is called an “electrogram,” is a more accurate means to discern ischemia as compared to the heart's signal as measured on the patient's skin which is the ECG.
The Fischell, et al patents as listed above discuss the storage of recorded electrograms and/or electrocardiogram data; however techniques to optimally capture the appropriate statistical electrogram and/or electrocardiogram data over days, weeks and months in a limited amount of system memory are not described.
The Reveal™ subcutaneous loop Holter monitor sold by Medtronic, Inc. uses two case electrodes spaced about 3 inches apart to record electrogram information. Recording can be triggered automatically when arrhythmias are detected or upon patient initiation using an external device. The Reveal is designed to record electrogram data and does not include the signal processing capability to track changes in the heart signal over an extended period of time. The Reveal also does not have the capability to measure ST segment shift. In fact, its high pass filtering and electrode spacing preclude accurate detection of changes in the low frequency aspects of the heart's electrical signal, which low frequency aspects are required for the detection of ischemia.
While pacemakers track the numbers of beats paced or not paced and pacemaker programmers can display the beat data in histogram format, pacemakers do not produce histograms of heart signal parameters related to the electrogram wave form. In other words, pacemakers track pacemaker operation but pacemakers do not measure or compute heart signal parameters of the beats in the electrogram signal, nor do they save the computed values of heart signal parameters in memory.
Pacemakers have been used to collect intramyocardial electrogram (IMEG) data for the purpose of using a decrease in electrogram QRS complex voltage as an indicator of the rejection of a transplanted heart. The expense, patient discomfort and inconvenience of endomyocardial biopsy to detect heart transplant rejection makes an electronic method highly desirable. The published paper “Clinical Heart Transplantation without Routine Endomyocardial Biopsy” by Warnecke, et al in the November/December 1992 issue of The Journal of Heart and Lung Transplantation showed that IMEG recordings made with a cardiac pacemaker have the potential to replace endomyocardial biopsy (EMB) as a diagnostic method to detect transplant rejection. Specifically, Warnecke et al showed that an 8% decline in IMEG voltage provided the best sensitivity and specificity as an indicator of potential acute moderate allograft rejection of a transplanted heart. Unfortunately, pacemakers are not designed to collect weeks or months of statistical data on electrogram voltage variations. The additional external support equipment needed to continually offload the raw electrogram data from a pacemaker is expensive and inconvenient to use.
The term “medical practitioner” shall be used herein to mean any person who might be involved in the medical treatment of a patient. Such a medical practitioner would include, but is not limited to, a medical doctor (e.g., a general practice physician, an internist or a cardiologist), a medical technician, a paramedic, a nurse or an electrogram analyst. Although the masculine pronouns “he” and “his” are used herein, it should be understood that the patient or medical practitioner could be a man or a woman. A “cardiac event” includes an acute myocardial infarction, ischemia caused by effort (such as exercise) and/or an elevated heart rate, bradycardia, tachycardia or an arrhythmia such as atrial fibrillation, atrial flutter, ventricular fibrillation, premature ventricular contractions or premature atrial contractions (PVCs or PACs) and the rejection of a transplanted heart.
For the purpose of this invention, the term “electrocardiogram” is defined to be the heart's electrical signal as sensed through skin surface electrodes that are placed in a position to indicate the heart's electrical activity (depolarization and repolarization). An electrocardiogram segment refers to electrocardiogram data for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. For the purposes of this specification, the PQ segment of a patient's electrocardiogram is the typically flat segment of a beat of an electrocardiogram that occurs just before the Q and R waves. For the purposes of this specification the ST segment of a patient's electrocardiogram is that segment of a beat of an electrocardiogram that occurs just after the S wave.
Although occasionally described as an electrocardiogram (ECG), the electrical signal from the heart as measured from electrodes within the body is properly termed an “electrogram” or intramyocardial electrogram (IMEG). For the purpose of this invention, the term “electrogram” is defined to be the heart's electrical signal from one or more implanted electrode(s) that are placed in a position to indicate the heart's electrical activity (depolarization and repolarization). An “electrogram segment” refers to a recording of electrogram data for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. For the purposes of this specification the PQ segment of a patient's electrogram is the typically flat, generally horizontal segment of an electrogram that occurs just before the Q and R waves. For the purposes of this specification the ST segment of a patient's electrogram is that segment of an electrogram that occurs just after the S wave. For the purposes of this specification, the term QRS voltage is defined as a measure of QRS complex voltage amplitude which may either be measured from Q to R, or S to R of a beat of the electrogram. For the purposes of this specification, the term QRS segment or QRS complex is that segment of the electrogram from the Q through the R and ending at the J point of the S wave. For the purposes of this specification, the terms “detection” and “identification” of a cardiac event have the same meaning. A beat is defined as a sub-segment of an electrogram or electrocardiogram segment which covers the electrical signal from the heart for exactly one beat of the heart and includes exactly one R wave. If the heart rate is 60 bpm, then the sub-segment of the electrogram that is exactly one beat would represent a sub-segment of the electrogram that is exactly 1.0 second in duration. For the purposes of this invention, the term “average value”, “average amplitude” or “average voltage” of any segment (viz., QRS complex, ST segment or PQ segment) of the electrogram shall be defined as meaning either the mean or the median of a multiplicity of measurements of that segment. It is also envisioned that in some cases both mean and median may be computed and will on occasion be described separately herein.
“Heart signal parameters” are defined to be any measured or calculated value created during the processing of one or more beats of electrogram (or electrocardiogram) data. Heart signal parameters are features of the electrogram derived from one or more measured values and include PQ segment average voltage, ST segment average voltage, R wave peak voltage, ST deviation (ST segment average voltage minus PQ segment average voltage), ST shift (ST deviation compared to a baseline average ST deviation taken at some prior time), average signal strength, T wave peak height, T wave average voltage, T wave deviation, QRS complex width, QRS voltage, heart rate and R-R interval. Counts of the number of arrhythmia related events such as PACs, PVCs and/or episodes of atrial fibrillation are not considered herein to be heart signal parameters as they do not directly result from a measured value derived from a beat of the electrogram. ST segment related heart signal parameters include, ST segment average voltage, ST deviation and ST shift.